The rate of rusting

Iron nails are kept in four conditions, and the rust on each is weighed every two days. You will make the program find the average rate of rusting in each condition, the condition that rusts fastest, and the ones where almost no rust forms. Then it draws all four.

The idea. Iron rusts only with both air and water, and salt makes it faster. The average rate is the rust gained divided by the time taken.

Your tasks.

1. Fill rates with the average rate for each condition, in mg per day.
2. Find fastest, the condition with the biggest rate.
3. Fill no_rust with the conditions whose final rust is less than $2\ \text{mg}$.